1. Field of the Invention
The present invention relates to a receiving apparatus, a receiving method, and a program, particularly to a receiving apparatus, a receiving method, and a program which can quickly obtain a base band signal with no frequency error.
2. Description of the Related Art
For a scheme of transmitting digital signals, the OFDM (Orthogonal Frequency Division Multiplexing) modulation is widely used. The OFDM modulation is a scheme that transmits signals in which digital modulated waves are multiplexed in multicarrier modulation, the digital modulated waves having a few tens to a few hundreds or a few thousands of orthogonal carrier wave frequencies.
In receiving signals using the OFDM modulation, there is a technique in which frequency errors contained in IF signals are detected by a carrier wave error detection circuit, whereby base band signals with no frequency error are obtained (for example, Patent Reference 1 (JP-A-2001-292123)).
FIG. 1 shows an exemplary configuration of a receiving apparatus 1 before to which the technique described in Patent Reference 1 is applied.
The receiving apparatus 1 has an antenna 11, a mixer 12, a local oscillator 13, a PLL (Phase Locked Loop) 14, a bandpass filter (BPF) 15, an A/D converting part 16, an orthogonal demodulating part 17, FFT (Fast Fourier Transform) computing part 18, a narrow band carrier wave frequency error detecting part 19, a wide band carrier wave frequency error detecting part 20, an adding part 21, and a controller 22.
The antenna 11 receives an RF signal at a center frequency fRF. The received RF signal is supplied to the mixer 12. The local oscillator 13 configured of a crystal oscillator oscillates the reference signal, the oscillated reference signal is multiplied by the PLL 14, and the oscillation signal at the frequency fLO is supplied to the mixer 12. The mixer 12 multiplies the supplied RF signal by the reference signal, and converts the RF signal into the IF signal at a predetermined center frequency fIF.
Harmonic components contained in the output of the mixer 12 are removed by the bandpass filter 15, and the remains are supplied to the A/D converting part 16.
Then, the A/D converting part 16 samples the IF signal supplied from the bandpass filter 15 from which the harmonic components are removed, and digitizes the IF signal. The IF signal digitized by the A/D converting part 16 is supplied to the orthogonal demodulating part 17.
The orthogonal demodulating part 17 is configured of an NCO (Numerically Controlled Oscillator) 31 which is a numeric value control the oscillator and multiplying parts 32-1 and 32-2. The orthogonal demodulating part 17 subjects the digitized IF signal to orthogonal demodulation, converts it into a base band signal of an in-phase component and a quadrature component (a complex signal that contains a real axis component referred to as an I-channel signal and an imaginary axis component referred to as a Q-channel signal), and supplies it to the FFT computing part 18 and the narrow band carrier wave frequency error detecting part 19.
The FFT computing part 18 subjects the OFDM time domain signal to FFT computation to extract modulated data for each subcarrier, subjects the supplied base band signal to OFDM demodulation to obtain the received signal, and supplies it to the wide band carrier wave frequency error detecting part 20 and a signal processing system in a later stage, not shown, (for example, an equalizer).
The narrow band carrier wave frequency error detecting part 19 computes narrow band carrier frequency error components showing the shift amount of the center frequency of the OFDM time domain signal after orthogonal demodulation by the orthogonal demodulating part 17. More specifically, the narrow band carrier wave frequency error detecting part 19 computes the shift amount of the center frequency having an accuracy of ±½ of the subcarrier frequency interval (for example, 4.14 Hz) or below, that is, the narrow band carrier frequency error component, and supplies it to the adding part 21.
In other words, the narrow band carrier wave frequency error detecting part 19 determines a correlation between the waveform in the guard interval portion and the waveform in the later half of the OFDM symbol (that is, the original signal waveform of the guard interval) for the OFDM time domain signal, and determines the order portion of the OFDM symbol based on the correlation. The function that expresses the determined correlation is a complex signal, and the phase component of the order portion of the OFDM symbol in the function is information about an accuracy of ±½ of the subcarrier frequency interval of the carrier frequency error component or below.
The wide band carrier wave frequency error detecting part 20 computes wide band carrier frequency error components showing the shift amount of the center frequency of the OFDM time domain signal for the OFDM frequency domain signal after FFT computation. More specifically, the wide band carrier wave frequency error detecting part 20 computes the shift amount of the center frequency in an accuracy of the subcarrier frequency interval (for example, 4.14 Hz), and supplies it to the adding part 21.
Generally, the OFDM signal contains a pilot signal called a CP (Continual Pilots) signal. The CP signal is a signal that always indicates a particular phase and amplitude, which is inserted into the subcarrier of a plurality of indexes in an effective symbol. The number of the CP signals contained in the effective symbol and the pattern of arranging the insertion positions of the CP signals are defined by standards.
The wide band carrier wave frequency error detecting part 20 subjects the OFDM frequency domain signal after FFT computation to differential demodulation two times between symbols temporally apart from each other, extracts the CP signal, and computes the carrier frequency error component of the OFDM signal by calculating how much the subcarrier position of the extracted CP signal is shifted from the original subcarrier position.
The adding part 21 adds the narrow band carrier frequency error component computed by the narrow band carrier wave frequency error detecting part 19 to the wide band carrier frequency error component computed by the wide band carrier wave frequency error detecting part 20 to compute the total shift amount of the center frequency of the base band OFDM signal. The adding part 21 supplies the computed total shift amount of the center frequency, that is, the frequency error information as an error component err to the NCO 31.
To the adding part 21, the controller 22 supplies the preset values of the oscillation frequencies of the NCO 31 in the first time reception operation after the power source of the receiving apparatus 1 is turned on and the reception operation after the channel is changed.
In the first time reception operation after the power source of the receiving apparatus 1 is turned on, the NCO 31 of the orthogonal demodulating part 17 supplies the signal at the originating frequency based on the preset value supplied from the adding part 21 to the multiplying parts 32-1 and 32-2. Then, after that, the NCO 31 receives an error component err from the adding part 21, and then generates an fc error correction signal that is a correction signal for correcting the error of the center frequency fc in the frequency tuning range and rises and drops depending on the error component err. For example, the NCO 31 controls the signal in such a way that it drops the oscillation frequency of the fc error correction signal when the supplied error component err is a positive value, whereas it raises the oscillation frequency of the fc error correction signal when the supplied error component err is a negative value. The NCO 31 controls the signal in this manner, whereby it generates such an fc error correction signal that the oscillation frequency becomes constant at the point at which the error component err is zero.
Next, the operation in the case in which the receiving apparatus 1 selects channels will be described.
When a user selects a channel, the PLL 14 generates a first channel originating signal at a frequency matched with the channel, and the mixer 12 performs frequency conversion so that the center frequency of a desired wave is fIF.
A channel originating signal frequency fLO1 is expressed by the following Equation (1), where the RF signal center frequency of the selected channel is fRF1.fLO1=fRF1+fIF  (1)
In the receiving apparatus 1 shown in FIG. 1, the scheme of upstream channel transmission is adopted, but it is the same in the case of downstream channel transmission.
The IF signal that the harmonic component is removed by the bandpass filter 15 is subjected to A/D conversion in the A/D converting part 16, and then it is subjected to orthogonal demodulation in the orthogonal demodulating part 17 with a second channel originating signal generated in the NCO 31. Suppose the second channel originating signal frequency to be generated in the NCO 31 is fNCO.
Generally, in the first time reception operation after the power source of the receiving apparatus 1 is turned on, the oscillation frequency fNCO of the NCO 31 is preset to the value expressed by the following Equation (2).fNCO=fIF  (2)
Frequency errors sometime occur in the PLL reference signal that is supplied from the PLL 14 to the mixer 12 because of the accuracy of the crystal oscillator used by the local oscillator 13. The frequency of the reference signal that is supplied from the PLL 14 to the mixer 12 in the case in which frequency errors occur in the reference signal is given by fref(1+e) where the ratio of errors is e. When frequency errors occur in the reference signal, the same errors occur in a first channel originating frequency that is obtained by multiplying the reference signal. The first channel originating frequency fLO1(1+e) where the reference signal is given by fref(1+e) is expressed by the following Equation (3).fLO1(1+e)=fLO1+e×fLO1  (3)
In other words, when a frequency error occurs in the reference signal where the ratio of errors is e, a frequency error occurs in the first channel originating frequency by e×fLO1. The frequency error also causes a frequency error in the IF signal. The center frequency fIF1 of the IF signal with the frequency error is expressed by the following Equation (4).fIF1=fIF+e×fLO1  (4)
In other words, the frequency error ferr1 contained in the IF signal is expressed by the following Equation (5).ferr1=e×fLO1  (5)
The narrow band carrier wave frequency error detecting part 19 and the wide band carrier wave frequency error detecting part 20 detect these frequency errors. Since the NCO 31 of the orthogonal demodulating part 17 is controlled in such a way that the oscillation frequency becomes constant at the point at which the error component err is zero in accordance with the error component err outputted from the adding part 21, the oscillation frequency fNCO is stabilized to fIF1 after a certain time period has passed.
In other words, the narrow band carrier wave frequency error detecting part 19 and the wide band carrier wave frequency error detecting part 20 detect the error component err that indicates the shift amount of the center frequency, and feed the error component err back to the NCO 31. The NCO 31 generates the fc error correction signal in which the oscillation frequency fluctuates in accordance with the error component err, and supplies it to the multiplying parts 32-1 and 32-2. Then, the multiplying parts 32-1 and 32-2 subject the fc error correction signal to complex multiplication for the OFDM signal of the base band, whereby the carrier frequency error is corrected to obtain the base band signal with no frequency error.
Then, in the case in which it is instructed to change the channel, the process steps described above are repeated.